The PcG silencing complex is considered as a master regulator of genomic programs, because it acts at different stages of development to define which sets of genes are active and which ones are quiescent 20, 28. The PcG system is composed of three repressive complexes (PRC1, PRC2 and PhoRC) working together to bring about gene silencing 20, 28. The PRC1 complex (Supplementary Fig. 3) includes a group of proteins termed CBX, because they contain a highly conserved chromodomain (CBX) at their amino terminus 29. The mammalian homologs of Drosophila polycomb proteins are CBX2, 4, 6, 7, and 8 28. In different cells, the PRC1 complex may contain different CBX proteins 30. The PRC2 complex includes four core subunits: enhancer of Zeste (EZH1, EZH2), suppressor of Zeste [SUZ12], and the WD40 domain proteins EED and P55 (RBBP4 and RBBP7) 28 (Supplementary Fig. 3). PhoRC, the third PcG complex, contains two proteins, Pho and its homologue Phol, which bind directly to DNA 28. In mammals, these proteins are encoded by the Yy1 gene (Supplementary Fig. 3), which has both repressive and activating functions 31.
